The present invention relates to tank overfill valves, and in particular to overfill valves for tanks used in the storage of toxic or flammable liquids such as those common to automotive service stations and the like.
A large number of toxic or flammable liquid storage tanks are in use today, one common use being the storage of gasoline or diesel fuel in underground tanks. Such liquid storage tanks are normally provided with an upright casing by which the tank is filed. In conventional installations in which the tank is buried, this casing runs from the tank up to the ground surface. A drop tube is received down the casing and is used as a fill tube for the tank. The drop tube normally extends down to about the bottom of the tank in order to reduce splashing of the product and creation of vapors when filling the tank. Two conventional methods are used as safety features that prevent vapors from collecting within the tank and pressurizing the tank as it is filled. In one, a fill hose from the supply truck is connected to the drop tube and the tank includes a separate vent outlet. The vent outlet is either connected to the supply truck by a recovery hose to recover vapors released while filling the tank or the vent vents the tank to the atmosphere. The other method uses a coaxial drop tube which is connected to the supply truck. When a tank having a coaxial drop tube is filled, a double headed valve is connected to the top of the coaxial tubes. One line from the tank supply truck is connected by the valve to the drop tube in order to introduce the liquid product and a second vapor recovery line is connected by the valve to the annular space between the casing and the drop tube. The vapors within the tank rise through the annular space and are recovered into the truck.
A common problem with such storage tanks is that the tank is often overfilled, resulting in spillage of the liquid product. Such spills present a severe health and environmental hazard in that the toxic liquid may reach the local ground water or otherwise enter into the ecosystem. The magnitude of this storage tank overfill problem is evidenced by the fact that many state and local governments are enacting regulations that require liquid storage tanks to be equipped with a device that will shut off flow to the tank when the tank is full. Although storage tank vents may be normally provided with ball float valves that raise to close the vent orifice, when the tank fills and closes the ball float valve, the tank becomes pressurized. When the fill line is disconnected this pressure forces product up out of the casing or fill pipe. Another problem is that in both the system using two separate hose connections or a single valve on a coaxial drop tube, product delivery from the truck is not halted until the tank, fill pipe and fill hose all are filled. When the fill hose is disconnected the product remaining in the hose will discharge onto the ground.
In an attempt to correct this overfilling problem devices have been developed that sound alarms or the like when the storage tank is full. However, such devices are relatively complex and expensive, and do not shut off flow to the tank. These electrical systems will fail if power is absent. Further, such devices do not readily retrofit to existing tanks.
The problem of providing an overfill shut off for liquid storage tanks, and particularly for those used by service stations, is complicated by the manner in which owners gauge whether the tank requires filling. Normally a long pole is lowered through the drop tube in order to determine the liquid level in the tank. Any shut off mechanism, such as ball float valves for tank vents, that obstruct the drop tube would prevent passage of the gauging pole and would require a different, more complex gauging system.